1. Field of the Invention
The present invention relates to digital data storage systems. More particularly, the invention concerns a data storage subsystem comprising a virtual removable media server (VRMS) that handles audit requests, whether directed to a physical volume or a virtual volume physically stored in cache or in a removable physical media item.
2. Description of the Related Art
Data storage libraries are one of the most popular types of mass storage system today. Generally, a data storage library connotes a great number of co-located portable data storage media, transported by a robotic mechanism between locations such as storage cells, read/write media drives, and input/output facilities.
These libraries have become popular for many reasons. First, the portable data storage media, usually magnetic tape or optical media, are typically inexpensive relative to other storage formats such as magnetic disk drives. Furthermore, libraries are easily expanded to accommodate more data, simply by adding more items of media and storage cells. Additionally, most libraries can be easily updated with new equipment as it comes onto the market. For example, a new model tape drive may be introduced to the library to supplement or replace the existing drives.
A typical library includes an expandable protective structure, constructed of various doors and panels, to house the library's drives, storage cells, robotic device, and related electronic equipment. The robotic device, called an "accessor" includes appropriate mechanical and electrical subcomponents to move side-to-side, lengthwise, and up-down within the protective structure. The accessor is usually managed by a component such as an automated library manager.
One new application of the data storage library is the virtual removable media server (VRMS). In a VRMS, data is stored in removable media items managed in a library, and also in a shared direct access storage cache. In particular, the cache stores newly received data from its host(s), and also reads stored data in response to requests from the host(s). The cache emulates removable media items by representing data in "virtual" removable media items, while providing many advantages not available with many removable media, such as high speed storage, data appendibility, and the like. All host requests are satisfied with reference to a media volume, whether they are ultimately satisfied from a physical media item in the library or from a "virtual" media item in the cache.
Additional information about tape emulation systems, many of which are well known in the art, is provided by the following references: (1) U.S. Pat. No. 4,467,421, entitled "Virtual Storage System & Methods", issued Aug. 21, 1984, and (2) U.S. patent application Ser. No. 08/707,891, entitled "Virtual Integrated Cartridge Loader for Virtual Tape Storage System", filed on Sep. 10, 1996 in the names of Carlson et al. The contents of the foregoing references are incorporated by reference into this application.
One source of VRMSs in today's marketplace is International Business Machines Corp., which manufactures the IBM 3494, using magnetic tape as its removable media. Although VRMSs such as these represent significant advances in the field of data storage libraries, International Business Machines Corp. has continually sought to improve the performance and efficiency of these systems. One area of particular focus is achieving compatibility and seamlessness between "virtual" and physical data storage systems.
Particularly, it is desirable for the VRMS to service host requests exactly as a data storage library would, without host awareness of any "virtual" media items. Thus, the VRMS should permit the host to interface with the VRMS as if the host were storing and retrieving data from a library of removable media items. Some host operations, however, inherently require reference to specific media items or volumes, and therefore have little or no applicability in VRMSs. Consequently, known VRMSs are not completely adequate for some applications due to certain unsolved problems.